A Traction City can be huge - an "Urbivore" with millions of inhabitants, to villages that move using small engines or sails.

Big Traction Cities are built in tiers like a wedding cake; the poor live next to the tracks and engines on the bottom, and the upper classes enjoy their mansions at the top of the city.

It was a dark, blustery afternoon in spring, and the city of London was chasing a small mining town across the dried-out bed of the old North Sea.

In happier times, London would never have bothered with such feeble prey. The great Traction City had once spent its days hunting far bigger towns than this, ranging north as far as the edge of the Ice Wastes and south to the shores of the Mediterranean. But lately prey of any kind had started to grow scarce, and some of the larger cities had begun to look hungrily at London...

...lookouts on the high watchtowers spied the mining town, gnawing at the salt flats twenty miles ahead... [the] mining town saw the danger and turned tail, but already the huge caterpillar tracks under London were starting to roll fostering faster. Soon the city was lumbering in hot pursuit, a moving mountain of metal which rose in seven tiers like layers of a wedding cake, the lower levels wreathed in engine smoke, the villas of the rich gleaming white on the higher decks, and above it all the cross on top of St Pauls Cathedral glinting gold, 2000 feet above the ruined earth.

Most cities have attachments called "Jaws" to catch prey and drag it into the Gut of the city. Here the prey is stripped, melted down and used as fuel. Its inhabitants then become members of the predator city, or even taken as slaves.

For a much earlier take on the basic idea, see the steam-powered houses from Henry Loudon's 1828 classic The Mummy! A Tale of the Twenty-Second Century.